Cryopumps are frequently used to remove gases from work environments and maintain them at high vacuum. Many processes require a near perfect vacuum for good results. Further, best process results and manufacturing efficiencies are often achieved where a vacuum is continuously maintained in the work space. In this way, uniform and repeatable processes may be performed without interruption. Recently, the increasing miniaturization of electronic components manufactured in vacuum has increased the sensitivity of those components to minute amounts of contaminants.
Vacuum manufacturing working environment base pressures before processing are typically below 5.times.10.sup.-5 torr. In most manufacturing operations, however, it is necessary to transfer materials into and out of the high vacuum work space. Conventionally, this may be done in two manners. The simplest method is to place the work material into the vacuum work chamber at atmospheric conditions. The vacuum work chamber is then evacuated prior to manufacturing. This period of evacuation is often lengthy and does not always achieve optimum conditions in the work chamber. An alternative to this approach is to utilize a vacuum load lock adjacent to the vacuum work chamber. The load lock is used to move material into and out of the work chamber while maintaining the work chamber at a high vacuum. In such a system raw material is placed in the load lock at ambient conditions after which the load lock is evacuated to an intermediate crossover pressure by a roughing pump. The load lock is then further evacuated to a high vacuum state by a small high vacuum pump, such as a cryopump. After the load lock space has been fully evacuated, it is opened to the work chamber and the raw material is transferred into the work chamber. Finished processed material is removed either through the same load lock or through a separate exit load lock in a "pass through" type system.
In typical applications of such a load lock assembly in a high vacuum system, the associated cryopump and load lock are subject to severe duty cycles. For example, a 40 cc volume load lock is cycled from 1 atmosphere to 1 torr every 3.6 seconds by the cryopump in a high throughput application. With such a high gas load on the cryopump, the operating temperature of the cryopump may potentially rise above 20.degree. Kelvin. Above 20.degree. Kelvin, the capacity of the cryopump's charcoal adsorbent for the three noncondensible gases, H.sub.2, Ne and He, is significantly reduced. If the charcoal is near a saturated condition, then previously adsorbed noncondensing gases are released from the cryopump's charcoal adsorbant as the charcoal warms up. Consequently, the noncondensible gases build up in the cryopump and prevent a high vacuum from being achieved in the load lock.
Accordingly, there is a need to prevent the build up of the three noncondensible gases in the load lock cryopump under such heavy gas loads in high throughput applications.